Received August 20, 2007
Thermal habitats harbor specialized communities of thermophilic
microorganisms, primarily prokaryotes. This review considers modern
systematics of prokaryotes and the place of thermophilic archaea and
bacteria in it. Among the existing hierarchical classifications of
prokaryotes, the bulk of attention is given to the one accepted in the
current second edition of Bergey's Manual of Systematic
Bacteriology, which is primarily based on 16S rRNA phylogeny and
phenotypic properties of the organisms. Analysis of the genomics data
shows that they on the whole agree with the 16S rRNA-based system,
although revealing the significance of the evolutionary role of lateral
transfer, duplication, and loss of genes. According to the
classification elaborated in the current edition of Bergey's
Manual, the prokaryotes currently culturable under laboratory
conditions are distributed among 26 phyla, two of which belong to the
domain Archaea and 24 to the domain Bacteria. Six phyla contain
exclusively thermophiles, and eleven phyla contain thermophiles along
with mesophiles, thermophiles being usually separated phylogenetically
and representing high-level taxa (classes, orders). In light of the
data on the topology of the 16S rRNA-based phylogenetic tree and some
other data, this review discusses the probable hyperthermophilic nature
of the universal common ancestor.
KEY WORDS: genosystematics, phylogenetic systematics,
hierarchical classification, prokaryotes, archaea, bacteria,
thermophiles